linux-kernel - Re: [PATCH 1/1] hugetlbfs: handle pages higher order than MAX

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Date:	Wed, 08 Oct 2008 11:17:59 -0500
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Andy Whitcroft <apw@...dowen.org>
CC:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Jon Tollefson <kniht@...ux.vnet.ibm.com>,
	Mel Gorman <mel@....ul.ie>,
	Nick Piggin <nickpiggin@...oo.com.au>
Subject: Re: [PATCH 1/1] hugetlbfs: handle pages higher order than MAX_ORDER

Andy Whitcroft wrote:
> When working with hugepages, hugetlbfs assumes that those hugepages
> are smaller than MAX_ORDER.  Specifically it assumes that the mem_map
> is contigious and uses that to optimise access to the elements of the
> mem_map that represent the hugepage.  Gigantic pages (such as 16GB pages
> on powerpc) by definition are of greater order than MAX_ORDER (larger
> than MAX_ORDER_NR_PAGES in size).  This means that we can no longer make
> use of the buddy alloctor guarentees for the contiguity of the mem_map,
> which ensures that the mem_map is at least contigious for maximmally
> aligned areas of MAX_ORDER_NR_PAGES pages.

But the memmap is contiguous in most cases. FLATMEM, VMEMMAP etc. Its only
some special sparsemem configurations that couldhave the issue because they
break up the vmemmap. x86_64 uses VMEMMAP by default. Is this for i386?

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